<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>
    .a1 {
      width: 100%;
      height: 500px;
      display: flex;
    }

    .left {
      flex-basis: 300px;
      height: 100%;
      background-color: rgb(96, 45, 45);
      margin-right: 100px;
    }

    .right {
      flex-grow: 1;
      background-color: purple;
      height: 100%;
    }
  </style>
</head>

<body>
  <div class="a1">
    <div class="left"></div>
    <div class="right"></div>
  </div>
</body>

</html>